1. Field of the Invention
This invention relates generally to magnetron sputtering, more particularly to high power impulse magnetron sputtering (HIPIMS), and most specifically to a method and apparatus for triggered HIPIMS at very low pressures.
2. Background of the Related Art
Physical vapor deposition, or PVD as it is more commonly known, has become a well known technique for the deposition of thin films. In general, with PVD processes, thin films can be deposited on various substrates by the condensation of a vaporized form of the material. The coating method can involve a purely physical process such as high temperature vacuum evaporation or plasma sputter bombardment of a target, or it may also include, in the case of reactive sputtering, a chemical reaction on the surface of the substrate with the sputtered material. Particular application of this technology is found in the fabrication of semiconductor chips, such as computer microprocessors and RAM chips where PVD is used to deposit thin films of such metals as aluminum and copper. Another application is the deposition of ultrathin films of noble metals such as gold, silver, copper, and platinum, on samples prepared for electron microscope imaging.
In one variation, magnetron sputtering techniques are employed in which a target material made from the metal or other substance to be sputtered is placed into a chamber, along with the substrate to be coated, and the chamber brought to near vacuum. To generate the plasma, a small amount of an inert process gas such as argon is introduced and a negative voltage applied to the target. The voltage can be applied in a continuous (direct current, dc) fashion. In some applications, especially with semiconducting or insulating targets, radio-frequency (RF) power is applied to the target, leading to negative self-bias of the target. Typically, negative voltages such as −300 V to −600 V are applied to the target. While applying the negative voltage, another member proximate the target is maintained at a positive potential relative to the target, thus serving as the anode of the discharge. In most cases the anode is a discrete part of the magnetron, with the process chamber in some cases serving as the anode. When power is applied, the argon gas ionizes and the argon ions attracted to the target, striking the target surface at high speed to dislodge (“sputter”) atoms of the target material.
In this sputtering process, secondary electrons are emitted from the target as well, these electrons captured by the magnetic field superimposed upon the target via magnets positioned behind the target. The magnets are commonly incorporated into the target support directly beneath the target to provide an arched magnetic field structure. The electrons, confined to an area adjacent the target, cause ionizing collisions with inert process gases near the target, enhancing the ionization of the plasma and thus further facilitating the sputtering process. In conventional sputtering, the electrons ionize the gas atoms and to a much smaller degree the sputtered atoms. The probability of ionization of those sputtered atoms is low because they leave the target surface typically with several electron-volts of kinetic energy, which corresponds to a high velocity of 1000 m/s or more. Therefore, any bias voltage applied to a substrate is of little effect because the sputtered atoms are electrically neutral.
As features on semiconductors have become smaller and smaller, it has become more important to deposit even thinner films onto the semiconductor substrate, and to obtain films of greater and greater purity, with a density approaching the theoretical limit of bulk density. In one known approach, the magnetron sputtering process is operated at a high power density such that some ions, mostly argon and to a much smaller degree ions of the sputtered material, are available for ion bombardment of a biased substrate. However, increasing the sputtering process power becomes limiting in that the power needed to create the higher density plasmas tends to cause overheating of the target, or even the entire magnetron unit, including its permanent magnets.
To address this problem, resort to the technique known as high power impulse magnetron sputtering (HIPIMS) has been developed. In this approach, power densities of roughly two orders of magnitude greater than those used for magnetron sputtering are employed. Here, a dc voltage is applied to the target for a short period of time, generally in the order of 10 μ and 50 μ seconds, though the pulsed dc voltage could be applied for longer times, such as up to 150 μ seconds, or more. The maximum pulse duty cycle is mainly limited by cooling considerations of the magnetron device. Typically in HIPIMS, the voltage is applied as a square wave with a duty cycle of about 1%. That is, the power is on for only about 1% of the time. With greatly enhanced power densities during the pulse on-time, ionization of sputtered atoms occurs to a much larger extent. By resort to impulse powering of the target, high voltages and high currents can be applied without causing the overheating and the generation of macroparticles typical of arcs.
With HIPIMS, the driving voltages need to be of sufficient amplitude so as to create a condition supportive of self-sputtering of the target material once discharge has been initiated. Thus, according to this technique, peak power densities (at voltages of between 500V and 1500 V) of over 100 W cm−2 to 5,000 W cm−2 are employed. Studies have also shown that densification of the growing film can be achieved by high ion-to-neutral ratios and high ion energy flux arriving at the substrate surface. Combined with deposition of atoms by magnetron sputtering, high flux plasmas are obtainable, leading to superior film properties such as a film density close to bulk material density, a smooth surface topology, and low resistivity.
With the high power densities employed with HIPIMS processing, the likelihood of ionization of sputtered atoms becomes significant. While the degree of ionization of sputtered atoms in conventional magnetron sputtering is only about one percent (1%), it can reach 50% and more with HIPIMS. For copper and silver, for example, ionization of the sputtered atoms approaches 100%. The greatest contribution to ionizing the sputtered material comes from electrons. The magnetized, energetic electrons are trapped by the arched magnetic field and therefore have a long path near the target with a high likelihood of colliding with the sputtered atoms to ionize them. Additionally, a smaller fraction may become ionized via collisions with excited argon. The latter mechanism works because the excited levels of argon have more energy than is needed to ionize sputtered metal atoms (in other words, it works because metals have a lower ionization energy than the energy stored in excited (but neutral) argon atoms.)
Notwithstanding the improvements afforded by HIPIMS, purity of the plasma remains an issue, due in part to the need for the introduction of an inert gas such as argon to both initiate and sustain the plasma. To date, argon has been the gas of choice since it is relatively inexpensive and does not react with the to-be-deposited material. For some applications, however, even with a pure noble gas, traces of the noble gas may find their way into the film coating. The presence of the argon, for example, can act as in impurity, as well as reduce the conductivity of sub-micron copper wires on chips due to argon-induced nano-voids and other defects.
Attempts have been made to address this problem, and in one approach, specific to copper and silver, argon was used to initiate the plasma; the discharge started with argon, but the gas supply turned off after the sputter process has begun, going into a sustained self-sputter mode. See for example, N. Hosokawa, T. Tsukada, and H. Kitahara, Effect of Discharge Current and Sustained Self Sputtering, Proc. 8th Int. Vacuum Congress, le Vide, Cannes, Frances, 1980, pp. 11-14, and W. M. Posadowski, Vacuum 46, 1017 (1995). While this approach can reduce the amount of gas captured by the deposited film, it does not eliminate it.